The TARDIS arrives on board a spaceship in orbit around a planet called the Sense-Sphere. The alien Sensorites have trapped the ship's human crew, Captain Maitland, Carol and John, in a state of semi-permanent paralysis. When the Doctor investigates, the aliens steal the lock mechanism from the TARDIS, thus trapping him and his companions. The Sensorites allow all but Maitland and Barbara down to the planet, where Ian falls ill from a sickness that has been wiping out the Sensorites. The Doctor finds a cure. His investigations into the cause of the sickness are hampered by the subversive activities of the City Administrator, but eventually he uncovers three deranged human survivors from a past expedition who have been adding deadly nightshade to the water supply.